The invention pertains to unipolar amplifier devices especially useful as amplifiers of signals in the microwave to millimeter wave range.
Field Effect Transistors (FET's) are commonly used as amplifiers of high frequency signals, most commonly in common source amplifier circuits. The common source configuration, however, has the inherent drawback that its input and output portions are theoretically impedance mismatched. For this reason, phase cancellation between the input and output occurs unless the length of the device in the direction of wave propagation (orthogonal to current carrier transit) is made a small fraction of a wavelength. This limitation on size inherently limits the power handling capacity of any such amplifier. The common gate amplifier configuration, however, can theoretically have impedance matched input and output portions, and is thus an excellent candidate for use as an amplifier of high frequency signals if provided with FET's that are internally impedence matched. Another limitation on amplifer power capacity is the inherent breakdown voltage of semiconductors, made worse by the peculiar property of some semiconductors to form weak cross bonds at the semiconductor's edge or interface with other non-lattice matched materials. These weakly held interface or surface electrons are much more easily raised from the valence to the conduction band, and in FET's cause breakdown at a much lower voltage than the inherent breakdown voltage of the bulk semiconductor of which the FET's channels are made. Another limitation on power capacity is the low thermal conductivity of many semiconductor materials.
A limitation on the dynamic range of devices of this kind is the inherent breakdown voltage between the FET gate and the FET channel; for isolated gate FET's, the voltage is the breakdown potential of the isolation material.
High frequency semiconductor amplifiers commonly use compounds of elements of column III and column V of the periodic table, such as gallium arsenide and indium phosphide, because of their extremely high maximum steady state drift velocities. Unfortunately, however, this inherent advantage is offset somewhat because the relationship between electric field potential and steady state drift velocity in both gallium arsenide and indium phosphide becomes negative shortly after the velocity peak. Thus semiconductor devices employing either gallium arsenide or indium phosphide require extremely highly doped regions along the path of carrier movement to insure that these carriers entering subsequent gain stages experience no electric field potential sufficiently higher than that corresponding to the maximum drift velocity. This requires slowing, then accelerating, carriers four times per rf cycle, partially offseting the value of the extremely high maximum drift velocity in these compounds, wasting considerable energy, and creating much excess heat. Moreover, because of the negative differential drift velocity versus electric field slope of these compounds, charge carriers at higher potential may, in fact, be moving slower than carriers at a lower potential, resulting in carrier bunching and the formation of localized dipole domains within the FET channel. To prevent such dipole domains from degrading device operation, the highly doped regions must be doped sufficiently to rapidly collect these domains and quench them.
As with all semiconductor devices, increasing tensile strength can simplify fabrication and make for a much more rugged and marketable product. Additionally, eliminating device parts, and thus the fabrication steps necessary to manufacture these parts, makes such devices simplier and easier to manufacture, and more reliable in the field because the probability of a fatal fabrication error increases with the number of steps necessary to fabricate a device.